1. Field of the Invention
The present invention relates to an illumination optical system, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
An exposure apparatus is employed to fabricate a semiconductor device using photolithography. The exposure apparatus illuminates a reticle (mask) with a light beam from an illumination optical system, and projects and transfers a pattern formed on the reticle onto a wafer (substrate) by a projection optical system.
Along with the recent advances in micropatterning of semiconductor devices, it has become indispensable to obtain a uniform illuminance distribution on a wafer in order to precisely transfer a fine pattern onto the wafer. Also, off-axis illumination (modified illumination) in which a reticle is illuminated with a shape optimized for the reticle pattern (for example, an annular shape or a quadrupole shape) is attracting a great deal of attention as one resolution enhanced technology for improving the depth of focus and the resolution of the exposure apparatus. A secondary light source formed by an optical integrator arranged in the illumination optical system serves as a pupil for the illumination target surface. Hence, oblique-incidence illumination can be realized by changing the light intensity distribution on the pupil plane (pupil intensity distribution) of the illumination optical system.
However, if the incident angle distributions (incident angular distributions) on the wafer (respective points on it) as the final illumination target surface have differences (light amount differences) in the horizontal direction (or the vertical direction), a positional shift occurs in a pattern transferred onto the wafer when the wafer is defocused from the focal plane of the projection optical system. As a result, differences (line width differences) occur in the horizontal direction (or the vertical direction) in the line width of a pattern transferred onto the wafer. Similarly, if the incident angle distributions on the wafer have light amount differences between the vertical and horizontal directions, line width differences occur between the vertical and horizontal directions in a pattern transferred onto the wafer. For these reasons, to precisely transfer a fine pattern onto a wafer using oblique-incidence illumination, it is necessary not only to adjust the light intensity distribution on the pupil plane of the illumination optical system into a desired shape, but also to make the incident angle distributions on the wafer as the illumination target surface uniform free from any image height differences.
To meet these requirements, Japanese Patent Laid-Open No. 2006-059834 proposes a technique of adjusting the incident angle distributions on the wafer by inserting an adjustment unit such as a filter having a predetermined reflectance distribution or transmittance distribution in the illumination optical system.
However, the prior art does not take account of the adverse effect of light (flare light) reflected by the adjustment unit such as a filter. Therefore, regions adjacent to the predetermined exposure region are exposed by the flare light reflected by the adjustment unit, resulting in deterioration in line width uniformity of a pattern transferred onto the wafer. Furthermore, because the prior art adjusts the incident angle distributions on the wafer by inserting one adjustment unit in the illumination optical system, it cannot adjust the incident angle distributions on the wafer with a sufficiently high accuracy for patterns rapidly becoming finer.